Active acoustic transmission loss box

Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Adjacent ear

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G10K 1116

Patent

active

056920539

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This present invention relates generally to noise or sound control and more particularly to the control of radiated sound from vibrating machinery by enclosing the machinery in what is termed an "active box or container". The purpose of the active box is to markedly reduce the radiation of the sound from the machine to observation points in the surrounding field, with a very lightweight, compact, non-airtight structure.


DISCUSSION OF RELATED ART

In many applications the radiation of sound from vibrating machines is an annoying noise problem. One technique which has been used in the past is to enclose the machine in a high transmission loss (TL) box in order to reduce the radiated sound (as described, for example, in U.S. Pat. No. 4,715,559, hereby incorporated by reference, herein and in "Noise and Vibration Control" by L. Beranek, 1988). These conventional boxes attenuate the sound transmitted through their walls by passive means. In order that the container be effective, i.e. strongly reduce the sound, it has to be both airtight and constructed from material which has a high density and thickness. These two conditions have a number of practical disadvantages. For example, the airtight condition implies that it would be extremely difficult to build an effective high TL container for applications which require air flow (e.g.a.c. units, compressors, etc.) or piping and wiring connections or ventilation for cooling. These requirements would imply significant holes through which the acoustic energy could leak. The high density material condition of course would imply that the box be extremely heavy and large in size, a problem which is exacerbated as the frequency of sound becomes lower.
Previous work has shown the extremely high potential of using active vibration inputs to structures to reduce the radiated sound from the structural vibration. Such work is described in "Apparatus and Method for Global Noise Control", U.S. Pat. No. 4,715,559, 1987, by C. R. Fuller and "Control of Sound Radiation with Adaptive Structures", Journal of Intelligent Material Systems and Structures, Vol. 2, pp. 431-452, 1991, by R. L. Clark and C. R. Fuller. The control inputs can be in the form of point force shakers or surface strain devices, such as piezoelectric elements, bonded to the surface of the structure. In order that the control approach be efficient and effective, the variable to be minimized has to be the radiated sound from the panel, measured, for example, by error microphones located in the radiated sound field as in Fuller. The controller format can be any control approach which adjusts the oscillating voltage inputs to the piezoelectric inputs, for example, in order to minimize the radiated sound observed at the error microphones. Polyvinylidene fluoride (PVDF) piezoelectric distributed sensors on the surface of a panel have been used in place of microphones to sense modes of the panel which are radiating efficiently to the far field such as that described in "Modal sensing of efficient acoustic radiators with polyvinylidene fluoride distributed sensors in active structural acoustic control approaches", J. Acoustical Society of America, pp. 3321-3329, June 1992, by Clark and Fuller. The work of Clark and Fuller, for example, demonstrates attenuations of the order of 20 dB of sound radiated from panels in the low frequencies (f<600 Hz) with only one or two active actuator inputs.


OBJECTS OF THE INVENTION

It is accordingly an object of the present invention to achieve high attenuation of radiated sound from a vibrating machine by enclosing it with an "active acoustic transmission loss box".
It is another object of the invention to achieve very high global (here global means throughout an extended area of "volume"), of sound with the above box constructed from very lightweight thin material, or to use the sides of the sound source itself to reduce radiated noise.
It is another object of the invention to achieve very high global sound attenuation with a container that is not airt

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